1. Field of the Invention
The present invention relates to a vibration motor for moving a vibration member and a contact member relative to each other with a frictional force, the vibration member being adapted to generate a vibration wave in an elastic member by applying an alternating signal from a driving circuit to a piezoelectric member serving as an electro-mechanical energy conversion element, and the contact member being in pressure contact with the vibration member, a vibration driving apparatus or vibration motor apparatus having the vibration motor and, more particularly, to a driving start system.
2. Related Background Art
There have been proposed a variety of vibration motors each for moving relative to each other, with a frictional force, a vibration member for generating a vibration wave in an elastic member and a contact member in pressure contact with the vibration member or vibration driving apparatuses such as an apparatus having a vibration motor. FIG. 10 shows a conventional rod type vibration motor.
FIG. 10 is a view showing the side surface of the rod type vibration motor, and the layout of voltage supply and output voltage extraction of voltage elements of a piezoelectric element constituting the rod type vibration motor. The rod type vibration motor has a vibration member 101 in which the piezoelectric element is sandwitched between rod like elastic members. A rotor 102 serving as a contact member is placed in pressure contact with the distal end portion of one elastic member by a pressure means (not shown). The rotor 102 rotates together with an output gear 103 so that a motor output can be extracted externally.
Each of the piezoelectric elements (a1, a2, b1, b2, and s1) is obtained by dividing one piezoelectric element plate into halves, which have different polarities. Semicircular electrode films are formed on the respective divided portions on the upper surface of the piezoelectric element, and an electrode film is formed on the entire lower surface of the piezoelectric element. An A phase piezoelectric element portion A for driving is composed of the piezoelectric elements a1 and a2 having opposite polarities through an electrode plate A-d. A B phase piezoelectric element portion B is composed of the piezoelectric elements b1 and b2 having opposite polarities through an electrode plate B-d.
An alternating signal is applied between a ground electrode plate GND-d and the electrode plate A-d of the A phase piezoelectric element portion A to excite lateral (bending) vibration within the plane formed by the axial direction of the vibration member and an axial direction crossing the polarized areas of the piezoelectric element plate. Similarly, an alternating signal is applied between an electrode plate GND-d and the electrode plate B-d of the B phase piezoelectric element portion B to excite lateral (bending) vibration within the plane formed by the axial direction of the vibration member and an axial direction crossing the polarized areas of the piezoelectric element plate. The A phase piezoelectric element portion A and the B phase piezoelectric element portion B have a phase difference of 90.degree.. When the bending vibrations of these portions are synthesized, surface particles on the driving surface move elliptically.
A vibration detection piezoelectric element S1 is disposed in the same phase with the piezoelectric element a2 in this vibration member. An insulating sheet (not shown) is arranged between a metal block forming one elastic member and the signal extraction electrode plate S-d contacting entirely the electrode of the vibration detection piezoelectric element S1, thereby being insulated from the GND potential. An output voltage corresponding to the vibration of the vibration detection piezoelectric element S1 can be directly output from the element S1. This output voltage is used to obtain a resonant frequency in accordance with its magnitude or phase difference from the driving voltage.
FIG. 11 shows a driving circuit for this vibration motor. A and B phase driving signals are applied to the electrode plates A-d and B-d of the A and B phase piezoelectric element portions A and B, respectively. A control circuit (to be referred to as a control microcomputer hereinafter) 11 drives and controls the motor. An oscillator 2 comprises a VCO (Voltage-Controlled Oscillator) for generating an alternating signal. A 90.degree. phase shifter 3 is connected to the oscillator 2. Switching circuits 4 and 5 switch the power supply voltage in accordance with the alternating signal from the oscillator 2 and the phase shifter 3. Inductance elements 6 and 7 are for impedance matching with the motor.
A phase difference detector 8 detects a signal phase difference .theta.(A-S) between the A phase driving signal and a vibration detection signal S. A speed detector 9 connected to the motor is an encoder or the like for detecting rotation of the motor and detecting the motor speed. The detection information is converted into a speed signal by a speed detection circuit 10, and the speed signal is input to the control microcomputer 11.
FIG. 12 shows the characteristics between the frequency and rotation number (speed) of the vibration motor. For normally starting a driving operating of the vibration motor, the frequency is gradually reduced from the higher side (f.sub.s) to reach a frequency f.sub.N0 serving as a target rotation number N.sub.0 so as to drive the motor more smoothly.
In this driving circuit, however, the conventional driving start method takes a long time until the motor rotation reaches a steady rotation number, and cannot obtain a short rise time.
Even if a frequency corresponding to a given rotation number is abruptly applied to the vibration motor, no quick driving start is attained because the vibration motor has hysteresis characteristics. The hysteresis characteristics are defined as a frequency area in which the motor can be started when the frequency is reduced from the higher frequency side but in which the motor cannot immediately be started when the frequency is increased from the lower frequency side.